The portable router is a versatile tool that is often under-employed. One important capability of the router that is not generally recognized or used is its ability to perform tasks that would normally be performed by a saw. The advantages of using a router instead of a saw include a more precise and finished edge on the cuts in the workpiece. Although routing may be initially slower than using a saw, the finished edge may save a further finishing step which may be required if a saw is used.
When routing a straight groove or a slot in a workpiece, a cutting line is first drawn on the workpiece to indicate where the groove or slot is to be cut. In order to guide a router bit along the cutting line, a fence or a straightedge is set parallel to and at a measured distance away from the cutting line and is then clamped or otherwise secured to the workpiece. A router is then guided along the straightedge. The measured distance is set so that the router bit cuts along the cutting line, as explained in further detail below. The minimum width of the resulting groove or slot is of course the cutting width or diameter of the router bit.
Conventionally, the groove or slot is cut along the cutting line on the side opposite the straightedge (this type of cut is referred to in this specification including the claims as an "outside" cut). Cutting the groove or slot along the cutting line on the same side as the fence or straightedge (this type of cut is referred to in this specification including the claims as an "inside" cut) is generally neither practiced, nor recognised in this field.
For an outside cut, the fence or straightedge will be clamped onto the workpiece at a distance away from the cutting line equal to the distance between the cutting edge of the router bit and the outer edge of the router base or, equivalently, the distance from the router axis of rotation to the edge of the router base less one-half the cutting diameter of the router bit. For example, for a router bit with a cutting diameter of 1/2-inch and a router having a circular base of 6-inch diameter, the proper setback distance for an outside cut would be 2 and 3/4-inches. This distance is commonly called the "offset" distance in the literature. However, a better term to use would be "setback" distance since the straightedge is "set" at a specific distance "back" from the cutting line.
The proper setback distance can be determined for each routing situation by measuring between the cutting edge of the router bit and the outer edge of the router base using a ruler or similar measuring implement. Typically, this measurement is then transferred to the workpiece by means of a measuring stick or ruler and is used to position a straightedge at the proper setback distance from the cutting line.
Alternatively, and less commonly, the fence or straightedge is positioned by using custom made spacers or gauges to directly measure a proper setback distance for a given router bit cutting diameter and a router base dimension. Examples of such spacers or setback gauges are found in the literature including Spielman, P., The New Router Handbook (New York: Sterling Publishing Co., 1993) at pp. 126, 128 and 129; and Spielman, P., Router Jigs and Techniques (New York: Sterling Publishing Co., 1988) at pp. 207, 208, 212, 213 and 214. These spacers are pre-cut from a suitable material to a width that matches the setback distance required for an outside cut for a specific router bit and router base.
A prior art spacer is used by placing one edge of the spacer along the cutting line and butting a straightedge along the opposite side of the spacer. Once the straightedge is in position, it is clamped or otherwise secured to the workpiece so that the straightedge may act as a stable guide for the router; the spacer is then removed.
A variation on the prior art spacer is a hinged spacer which comprises two flaps--a base flap and a spacer flap--hinged together along a lengthwise edge. The hinged spacer is illustrated in Spielman, P., Router Jigs and Techniques (New York: Sterling Publishing Co., 1988) at p. 207. The width of the spacer flap is equal to the distance from the router axis of rotation to the edge of the router base less one-half the cutting diameter of the router bit (i.e., the proper setback distance for an outside cut). The width of the base flap is undefined. In operation the hinged spacer measures a proper setback distance for an outside cut by lining up, in unfolded position, the free (i.e. unhinged) edge of the spacer flap against a cutting line on a workpiece, then folding the spacer flap onto the top of the base flap. A straightedge is formed by the hinged edge of the base flap. A router guided along the hinged edge of the base flap will thus perform an outside cut along the cutting line.
While these prior art spacers are appropriate for use in some routing situations, it is sometimes inconvenient or simply not possible to perform a desired outside cut using them. For example, when a cutting line is drawn to designate the outer edge of a panel that is to be cut from a large sheet of material, that panel can be cut by means of outside cuts if the panel is wide enough to enable a fence or straightedge to be clamped onto it for routing an outside cut. However, if the desired panel is not wide enough for the fence or straightedge to be clamped onto it, then an unconventional inside cut can be used, as detailed further below. In addition, because of their relative bulk, the prior art spacers are difficult and unwieldy to use in tight work spaces or on smaller workpieces. Furthermore, customizing and storing a complete set of these prior art spacers for common router bit cutting diameters may be expensive and impractical, particularly as such spacers are commonly provided as a single, long spacer.